Many cameras are known in the art, in which at least one of the exposure factors may be set either manually or automatically, the automatic setting being in accordance with prevailing light conditions as sensed by the automatic mechanism. As well known in the art, there are three exposure factors, one being the shutter speed (that is, duration of each exposure), another being the diaphragm aperture, and the third being the film sensitivity or film speed. For purposes of the present discussion, the two basic exposure factors are shutter speed and diaphragm aperture, which frequently need to be varied from one exposure to the next, whereas the factor of film sensitivity is relatively constant, as it is always the same for any one loading of the camera with a roll of film, and needs to be changed only when the camera is loaded with a fresh roll of film having a different sensitivity or film speed.
In many cameras according to the prior art, there is provision for indicating, in the viewfinder of the camera, the factor value which would be set by the automatic setting mechanism, but no provision for easy and quick comparison of this value with the actual value which the user sets manually, when he prefers manual control of the camera. Thus the user must put the viewfinder of the camera to his eye in order to see, in the viewfinder, the exposure value determined by the light-responsive mechanism of the camera, and then must remove the camera from his eye to look at the graduation on the manual setting knob or ring, to see the value which he has set manually, and must then return the camera to his eye to complete composing the picture and making the exposure. This is both awkward and time-consuming.
It has also been suggested that, when switching over from automatic operation to manual operation, the same indicating means visible in the viewfinder to indicate the value set automatically could also be used to indicate the value set manually, so that the user would not have to remove the camera from his eye and look at the graduations on the setting knob or ring. However, in that case the automatic value would no longer be visible, so that the operator would not have any basis for comparison of the exposure value which he decides to set manually with the exposure value which would be set by the automatic mechanism if he were to take advantage of the automatic setting.
An object of the present invention is to provide means for indicating to the user of the camera, preferably in a position visible through the viewfinder, both the exposure value factor which the automatic mechanism of the camera would select on the basis of prevailing light conditions, and the exposure value which the user selects manually. By providing both indications concomitantly or substantially concomitantly, the user can thus see at a glance whether he is setting the controls to the same value which the automatic mechansim would select, or to a different value, and can also see how much difference there may be between the value he wishes to select automatically and the value which the automatic mechanism of the camera would select. This makes it easy for the photographer to obtain exactly the effect he desires, enabling him quickly to select, for example, a shutter speed somewhat faster or somewhat slower than the speed which would be selected by the automatic mechanism, or a diaphragm aperture somewhat smaller or somewhat larger than the aperture which would be selected by the automatic mechanism.
Another object of the invention is to provide for mechanism which will achieve the above mentioned object in a manner which is both simple and comparatively inexpensive, utilizing so far as possible the same parts or components for the manual value indication as are used for the automatic value indication.
Still another object is to achieve these results in such a way that the user of the camera can readily observe the two values (automatic and manual) substantially simultaneously or concomitantly, the two values being given such a way that they are readily distinguishable from each other.
According to one embodiment of the invention, the values are indicated by light sources such as light emitting diodes (LED) visible in the viewfinder, different light sources indicating different values. The automatic and manual values may be distinguished by flashing the light sources, the frequency and duration of flashing for the manual indication being materially different from the frequency and/or duration of flashing of the automatic indicator. Other ways of distinguishing between the two indications are by the use of different colors of light, or materially and noticably different intensities of light. Although these are possibilities within the scope of the invention, it is preferred to use the same color and same intensity for both indications, as this permits the economy of using the same LED for both indications, distinguishing one from the other by readily observable differences in frequency and/or duration of the respective flashes.
According to a preferred embodiment of the invention, the light sources are connected to the outputs of a control circuit, which outputs are actuated selectively according to the manually selected or automatically formed exposure factor values (shutter speed or diaphragm aperture), and the input of the control circuit is connected with an electronic changeover switch which, in the case of manual camera operation, charges the input of the control circuit alternately with a signal derived from the manually selected exposure value and a signal derived from the automatically formed exposure value.
According to another aspect of the invention, the electronic changeover switch is controlled in such a way that, when the camera is set for automatic operation, the input of the control circuit is constantly charged with a signal derived from the automatic exposure factor. Thus during automatic operation of the camera, the value of the particular exposure factor (shutter speed or diaphragm aperture) which is being dealt with is signalled to the operator by constant illumination of the particular light source (e.g., a LED) allocated to or intended to indicate this particular exposure value.
According to another feature of the invention, the electronic switch is controlled by a pulse generator which emits a pulse sequence having a keying ratio which is materially different from one, that is, very much greater or very much less than one. In the case of manual operation of the camera, two flashing signals with different flash durations can be seen in the viewfinder, and thus it is possible to distinguish easily between the manually selected value and the theoretical optimum value determined by the automatic mechanism.
In accordance with a still further development of the invention, the electronic changeover switch is provided in a very simple manner, having two transistors one of which is connected between the input of the control circuit for the indicator device and the output of the circuit arrangement for the automatic value determination, the other transistor being connected between the input of the control circuit and the output of the manual selector, the bases of the two transistors being each connected with an output of a strongly asymmetrical astable multivibrator.
In a further feature of the invention, the multivibrator is switched on when the exposure value is to be determined manually, and is switched off when the exposure factor is to be determined automatically, and the transistor connected with the output of the circuit for automatic operation is kept constantly conductive. Thus during manual control of the exposure factor, the input of the control circuit can be charged alternately with signals derived from the manually selected and the automatically formed exposure values, while in automatic operation of the camera only the signal derived from the automatically formed exposure value is present uninterruptedly at the input of the control circuit.
Another feature of the invention relates to the arrangement of the changeover switch. This switch is arranged in the circuit of the multivibrator in such a way that, when set for manual operation, the switch connects the multivibrator with a voltage source, and when set for automatic operation, it connects the base of the transistor connected with the circuit arrangement for automatic operation to ignition potential, and connects the base of the transistor connected with the exposure time or diaphragm aperture selector to blocking potential. In this way the circuitry expense in the switching over of the camera into one or the other type of operation can be kept low for the indicator device.
In the preferred embodiment of the invention, the indicating light sources are formed as luminous diodes or light emitting diodes (LED) arranged in a line. Thus it is possible to form the indicating device with minimum dimensions, arranged in a line along one edge of the viewfinder image area in the camera.